This invention generally relates to devices for cleaning heat exchanger vessels, and is specifically concerned with an improved pressure pulse cleaning apparatus for loosening and removing sludge and debris from the secondary side of a nuclear steam generator.
Pressure pulse cleaning devices for cleaning the interior of the secondary side of a nuclear steam generator are known in the prior art, and have been disclosed in U.S. Pat. Nos. 4,655,846 and 4,699,665. Such devices generally comprise a gas-operated pressure pulse generator having an outlet that is mountable in communication with the interior of the secondary side of the steam generator. The purpose of these devices is to loosen and remove sludge and debris which accumulates on the tubesheet, heat exchanger tubes and support plates within the secondary side. In operation, the secondary side of the generator is first filled with water. Next the outlet of the gas-operated pressure pulse generator is placed into communication with the water, such as by a nozzle which may be formed from either a straight section of conduit oriented horizontally over the tubesheet of the generator, or a pipe having a 90 degree bend which is oriented vertically with respect to the tubesheet. Finally, pulses of gas pressurized to between 50 and 5000 pounds per square inch are generated out of the nozzle of the pressure pulse generator. The succession of pressure pulses create shock waves in the water surrounding the tubesheet, the heat exchanger tubes and support plates within the secondary side of the generator. These shock waves effectively loosen and remove sludge deposits and other debris that accumulates within the secondary side over protracted periods of time.
While the cleaning devices disclosed in these patents represent a major advance in the state of the art, the applicants have found that there are limitations associated with these devices which limit their usefulness in cleaning nuclear steam generators. However, before these limitations may be fully appreciated, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
In the secondary side of such steam generators, the legs of the U-shaped heat exchanger tubes extend through bores in a plurality of horizontally-oriented support plates vertically spaced from one another, while the ends of these tubes are mounted within bores located in the tubesheet. The relatively small, annular spaces between these heat exchanger tubes and the bores in the support plates and the bores in the tubesheet are known in the art as "crevice regions." Such crevice regions provide only a very limited flow path for the feed water that circulates throughout the secondary side of the steam generator. The consequent reduced flow of water through these regions results in a phenomenon known as "dry boiling" wherein the feed water is apt to boil so rapidly in the crevice regions between the heat exchanger tubes and the bores in the support plate and tubesheet that these areas can actually dry out for brief periods of time before they are again immersed by the surrounding feed water. This chronic drying-out of the crevice regions due to dry boiling causes impurities dissolved in the water to precipitate out in these regions. The precipitates ultimately create sludge and other debris which can obstruct the flow of feed water in the secondary side of the generator to an extent to where the steam output of the generator is seriously compromised. Moreover, the presence of such sludges is known to promote stress corrosion cracking in the heat exchanger tubes which, if not arrested, will ultimately allow water from the primary side of the generator to radioactively contaminate the water in the secondary side of the generator.
To remove this sludge, many other types of cleaning devices were used prior to the advent of pressure pulse cleaning devices. Examples of such prior art cleaning devices include ultrasonic wave generators for vibrating the water in the steam generator to loosen such debris, and sludge lances that employ a high-powered jet of pressurized water to flush such debris out. However, such devices were only partially successful in achieving their goal due to the hardness of the magnitite deposits which form a major component of such sludges, and the very limited accessibility of the crevice regions of the steam generator.
Since its inception, pressure pulse cleaning has been a very promising way in which to remove such stubborn deposits of sludges in such small spaces, since the shock waves generated by the gas operated pressure pulse operators are capable of applying a considerable loosening force to such sludges. However, the applicants have found that the devices disclosed in both U.S. Pat. Nos. 4,655,846 and 4,699,665 have fallen short of fulfilling their promise in several material respects. For example, research conducted by the applicants indicates that the orientation of the nozzle used to introduce the pulses of gas into the secondary side significantly affects the peak stresses applied to the tubes closest the nozzle, and that prior art nozzle geometrys fell far short of minimizing these stresses. Still another shortcoming observed by the applicants was the lack of any means to remove dissolved ionic species from the water during such prior art cleaning processes. Such ionic species, if not removed, are capable of precipitating out in the form of new sludges after the termination of the pressure pulse cleaning process if no provision is made to remove them. Additionally, applicants observed that if no provision is made to remove fine particulate matter from the water during the pressure pulse cleaning method, these fine particles of sludge are capable of settling onto the tubesheet and densely depositing themselves into the crevice regions between the tubesheet and the legs of the heat exchanger tubes, thereby defeating one of the purposes of the cleaning method. Finally, the applicants have observed that the relatively rapid pulse frequency taught in the prior art does not give the nozzle and manifold of the pulse generator sufficient time to fill back with water, and thus leaves pockets of shock-absorbing gas in the pulse generator which limits the efficacy of later generated pulses in generating sludge-loosening shock waves. Clearly, what is needed is an improved pressure pulse cleaning apparatus which overcomes the limitations associated with prior art pressure pulse cleaning devices and which is imminently practical for use in the secondary side of a nuclear steam generators.